Devices for detecting light have been known in practice for many years and are used, for example, in laser scanning microscopes. In this case, corresponding devices which pick up the detection signal from the sample to be examined under a microscope are of central importance for image quality. This applies above all to comparatively weak detection signals such as are typical in (confocal) fluorescence microscopy, SHG microscopy or Raman microscopy.
For light detectors, two characteristic variables are of particular significance: the detector noise and the quantum yield, i.e. the detection efficiency. The quantum yield describes the proportion of the light incident on the detector that actually generates a useable electrical signal. The noise denotes the background electronic signal, which is overlaid in an interfering manner on the actual detection signal. The ratio of these two variables, what is known as the signal-to-noise ratio (SNR), is one of the central characteristic variables of a light detector.
In practice, for many years photomultipliers (PMT) have been the dominant light detectors in laser scanning microscopy. In comparison with semiconductor-based detectors—e.g. photodiodes—PMTs have a lower quantum yield. Due to their low noise, however, they offer a very good SNR. Furthermore, improved variants comprising a GaAsP (gallium arsenide phosphide) layer as a light-sensitive medium have been available in recent years.
Furthermore, it has been known for some years to use alternative semiconductor detectors in fluorescence microscopy. In this case, above all single-photon avalanche diodes (SPADs) play a major role. The SPADs operate in Geiger mode.
Here, a reverse bias which lies just above the breakdown voltage is applied to the SPADs. In this case, the breakdown voltage is a few hundred Volts.
In this mode, an absorbed photon generates an electron-hole pair in the semiconductor, which is accelerated by the strong electric field and carries out further collision ionizations. This process continues in an avalanche-like manner and triggers a measurable charge avalanche that is amplified by a factor of several millions. Thus, individual, absorbed photons can be measured, so that these detectors can be used for the measurement of extremely low light quantities, as are typical in fluorescence microscopy, for example.
A single photon leads to an electric discharge that is measured in the form of a short voltage pulse. In this case, there are two fundamental measuring modes. In the digital measuring mode, the voltage pulses are counted, the rising voltage flank serving as a triggering counting signal. Alternatively, the charge can be integrated in the analogue measuring mode by means of a measuring resistance, and the grouped charge quantity of all the pulses used as the measurement signal. Typically, the integrated charge quantity of all pulses in a specified time interval (pixel exposure time) is then digitized by an analogue-digital converter for further digital processing.
Regardless of the selected measuring mode, in the SPADs it is problematic that saturation of the signal occurs. In concrete terms, with an increasing amount of light incident on the detector, the measurement signal no longer increases to the same extent. The desired linear relationship between the input and output signal therefore no longer exists. Saturation occurs as a result of the fact that, during an avalanche discharge of the SPADs, a further absorbed photon cannot trigger a simultaneous second avalanche. Thus, following the initiation of a pulse, a dead time of the SPADs occurs during which detection cannot take place. This dead time corresponds to the time that is required to replenish in the semiconductor the charge carriers depleted during the avalanche discharge. Large quantities of light in which a plurality of photons arrive during the dead time can therefore no longer be fully picked up and the detector shows a non-linear characteristic saturation curve.
Since the low dynamic range of the detectors resulting therefrom at a maximum count rate of some 106 to 107 photons per second represents a problem of these highly sensitive detectors, over recent years, SPAD arrays have been developed. These are available from Hamamatsu Photonics K.K. under the name MPPC (multi-pixel photon counting) detector, for example. In the literature, these detectors are also referred to as silicon photomultipliers (SiPM), inter alia. The function of corresponding detectors is described, for example, under https://www.hamamatsu.com/resources/pdf/ssd/mppc_techinfo_e.pdf.
The basic principle of an SPAD array consists in that a plurality of individual SPADs are connected together in parallel to form a field. If a photon is incident on a single SPAD, then due to its dead time it is no longer sensitive for a period of typically several nanoseconds. Other SPADs, on which a further photon is incident within this time or at the same time as said other photon, can, however, detect this and generate a measurable charge pulse. Consequently, a pulse sequence can occur at the detector output which has a higher count rate than with a single SPAD.
It is thus known from the prior art to divide the entire detection light amongst a plurality of SPADs connected in parallel. This has the advantage that individual SPADs have only a fraction of the detection light applied to them and thus saturation of these SPADs occurs at a later point. Furthermore, during the dead time of an individual SPAD, further reception-ready SPADs are available. The dynamic range of these detectors is therefore significantly increased, depending on the number of SPADs connected in parallel. Commercially available SPAD arrays have, for example, 20×20 or more SPADs.
However, the known SPAD arrays are problematic in that they exhibit saturation behavior. Saturation can arise if too many photons are incident on the same SPAD within the dead time. In this case, the saturation is similar to the saturation of an individual SPAD. Furthermore, saturation can also take place in the digital detection mode if the—rising—trigger flanks of a pulse during a previous pulse do not lead to the renewed triggering of the digital counter since their voltage level lies above the voltage threshold for counter triggering—what is known as the trigger level—and is therefore not detected by the counter unit as a pulse flank.
Both the effects mentioned also lead to a saturation of SPAD arrays. It is known in practice that, for example, above approximately 108 incident photons per second, the output signal—the number of electric discharges—assumes an almost constant value, so that precise measurement of the light quantity is no longer possible. Provided the characteristic curve—specific for a particular detector design—is known, it can be linearized by computational correction. In the region of almost complete saturation, however—for example, somewhat above 1011 photons per second—a sufficiently precise computational correction is no longer possible. Since a characteristic curve in the analogue measuring mode shows an almost identical shape to that in the digital measuring mode, no further distinction will be made below between the different saturation causes.
Although the parallel connection of a plurality of SPADs to form an SPAD array represents an improvement in the saturation problem, this is however still present. Thus, the dynamic range of known SPAD arrays is still below the dynamic range of photomultipliers, which are therefore still used for the detection of small light quantities even though their detection efficiency is poorer than that of SPAD detectors.